Large amounts of kaolin find use as pigments and as filler material in paper coating compositions. Kaolinite, the principal constituent of kaolin clay (or kaolinitic clay), is a white clay mineral that imparts brightness, gloss, smoothness and other desirable properties to the surface of coated paper, paper board, super-calendared paper, and other paper related products.
Kaolin is a clay, mined from clay deposits found in various locations around the globe. For instance, Kaolin ores are mined today from deposits located in the United Kingdom, France, Germany, and the Czech Republic. Kaolin deposits present in these locations are generally of the primary type. Primary kaolin deposits were formed through the weathering of feldspar, a group of rock forming minerals that comprise more than half the volume of the earth's crust. Kaolin clays are also mined today from deposits located in Brazil and the Southeastern U.S.A. Kaolins mined from these locations are generally of the secondary (or sedimentary) type. Secondary deposits are formed by sedimentation, a geological process that deposits kaolin within sedimentary rock.
Both types of kaolin clay deposits invariably contain coarse and fine impurities that must be removed prior to use. Coarse impurities, which can comprise more than half the crude volume, include +325 mesh-sized particles of quartz, feldspar, mica, or tourmaline. Fine impurities include silt- and clay-sized non-kaolin minerals, fine sand, and minerals. Sedimentary kaolin often contains titania mineral impurities, such as anatase and rutile. These titania minerals can contain lattice substituted iron that imparts a brown to yellow color. Along with titania, sedimentary kaolin often contains a small percentage of associated iron oxides and iron sulfides. Iron oxide minerals, such as hematite and goethite, impart a dull yellow-to-red color. Iron sulfide minerals, such as marcasite and pyrite, typically occur in kaolins containing organic matter. Iron sulfide minerals may occur as +325 mesh sized nodules, or be disseminated with organic matter to impart a gray color. Even in small quantities, these impurities can reduce the brightness and whiteness of pigments derived from kaolin. Because bright, white pigments are generally preferred over dull, yellow pigments, great effort has been made to develop processes to remove such impurities prior to use.
Wet mineral separation processes have been designed to remove impurities, or otherwise improve the characteristics of final kaolin products. In general, these processes are called beneficiation. Some beneficiation processes, including degritting and desanding, are designed to the remove sand, coarse silt, and other coarse particles that would detract from the physical properties of the final pigment. Other beneficiation processes are designed to remove fine impurities, or to improve the color, texture, or rheological properties of the final product.
The wet process is accomplished by first mixing crude clay with water to form aqueous mineral slurries, called slip. Where kaolin is mined dry, slip is formed by mixing—or blunging—dry or moist crude clay with water to produce an aqueous suspension having between 35% and 70% solids. The clay slurry is then degritted by passing it through a series of drag boxes, bucket-wheel desanders, hydrosizers, hydrocyclones, sieves, and/or screens, to separate the coarse materials. Alternatively, or subsequent to desanding, the crude slurry is left to stand for a period of time in settling bowls or thickeners, to allow the coarse particles to separate from the fine in a process called sedimentation. See for instance the Background section of Pruett et al., U.S. Pat. No. 6,149,723, which is incorporated herein by reference.
To increase the efficiency of degritting processes, it has long been known to deflocculate the clay slurry prior to degritting. Flocculation is the result of kaolin particles' tendency to adhere to one another and form aggregates, agglomerates, or flocs. Kaolinite particles will flocculate in strongly to weakly acidic environments, as positive and negative charges, present on the edges and faces of the kaolin particles, bring kaolin particles together. Flocculation causes the viscosity of clay-water slurries to rise. It also interferes with the degritting process by inhibiting coarse particles from settling out, causing flocs of fine clay particles to settle with coarse particles, and causing fine particles attached to coarse particles to be removed with the coarse particles. Deflocculation causes the particles to disperse. Dispersion helps liberate the particles and facilitate size and mineral separation.
Deflocculation is accomplished by mixing chemicals into the slurry that raise the pH (typically to 6 or higher). The increased alkalinity renders the kaolinite surfaces predominately negatively charged, and decreases the attractive forces among the particles. Deflocculation is aided further by the addition of dispersing agents, which minimize Van der Waal attraction among the particles.
Additional beneficiation of dispersed kaolin slip typically precedes or follows degritting, and accomplishes a variety of purposes. The selection of the beneficiation process used depends on the type of crude used, and on the specifications required of the final refined product. For instance, one may improve performance or brightness of a kaolin pigment through particle size classification. Classification processes select, or fractionate, particles conforming to certain ranges of particle shapes and/or sizes, called particle size distributions (psd). Classification, may be accomplished by, e.g., centrifugation as described, for example, by Hughes et al., U.S. Pat. No. 4,018,673, incorporated herein by reference. One may improve the color through various other beneficiation processes, including floatation (U.S. Pat. No. 3,655,038), froth floatation (U.S. Pat. No. 4,472,271 and EP 591406), magnetic separation (WO 9850161), reduced acid leaching or alkaline bleaching (U.S. Pat. No. 4,650,521), and selective flocculation. See U.S. Pat. Nos. 3,701,417, 5,685,900, 5,535,890, 4,227,920, and 6,068,693, incorporated herein by reference, or for variations thereof, Provisional U.S. Application No. 60/240,861.
Selective flocculation is a preferred way to remove titania, when present as an impurity. In some selective flocculation methods, kaolin containing titania (and other fine impurities) is mixed with additives that cause the titania to flocculate, settle to the bottom of a thickener or settling bowl, and leave the product kaolin to be recovered from the supernatant in dispersed form. See, for example, U.S. Pat. Nos. 3,701,417 and 6,068,693. The efficiency of selective flocculation can be improved by performing the reverse: i.e., by flocculating and recovering the kaolin component, and leaving the impurities to separate in the aqueous supernatant. See U.S. Pat. Nos. 5,535,890, and 4,227,920. In such processes, the kaolin is first dispersed by adding chemicals that increase its alkalinity. Dispersing agents are also optionally added. High molecular weight polymers are then added to the dispersed aqueous kaolin suspension. The suspension is then flocculated. During flocculation, the high molecular weight polymers adhere preferentially to kaolin, and do not attach to titania and other impurities. This facilitates titania separation. After separation, the flocculated kaolin portion is deflocculated chemically, e.g., by ozonation, chemical dispersants or both, or mechanically, e.g., by high shear wet milling, leaving a dispersed kaolin slip substantially free of titania and other impurities. See U.S. Pat. No. 5,685,900, incorporated herein by reference.
For crudes having iron oxides as an impurity, a beneficiation process called reduced acid leaching has been used to improve the color of the beneficiated product. According to long established practice, reduced acid leaching requires acidifying a dispersed (optionally degritted and beneficiated) clay slip to a pH range below 5, and typically below 3, to flocculate the clay particles. A chemical reducing agent is then added to the flocculated kaolin suspension. The reducing agent reacts with iron oxides, irreversibly converting them from water-insoluble ferric (iron III) oxides to the water-soluble ferrous (iron II) form. See, e.g., GB 1,043,252. Once solubilized, the iron was thought to leach into the aqueous portion of the clay slurry along with other water-soluble salts, and thereby separate from the flocculated kaolin. See generally U.S. Pat. Nos. 1,791,959; 2,339,594; 3,937,632; 4,002,487; and G.B. 1,043,252, incorporated herein by reference.
While most beneficiation processes may be carried out in any expedient order, it was thought necessary to save reduced acid leaching until the final stages of beneficiation, and after selective flocculation. This is because reduced acid leaching was performed on acid-flocculated clay slip and must be followed by filtration to remove salts to raise solids concentration for drying. Moreover, because selective flocculation requires flocculation of selected minerals from a fully dispersed slurry, it is necessary to carry out selective flocculation in alkaline conditions. (WO 00/68160 teaches a process for the selective flocculation of kaolinite, followed by ozonation, which is carried out in highly alkaline conditions, at pH levels above 9, preferably 11-11.5.) By contrast, iron removal by reductive leaching was thought to require acidic conditions that promote flocculation to enable removal of solubilized iron by filtration.
These constraints increase the cost of beneficiation for several reasons. The divergent pH requirements for reduced acid leaching and most other beneficiation processes made it impossible to combine them. Adjusting the pH to more alkaline levels required for selective flocculation of kaolinite and reverse froth floatation of anatase requires adding costly chemicals. Adjusting the pH again to acidic levels for acid reductive leaching, then again to a neutral pH prior to drying and sale, requires adding still more costly and environmentally harmful chemicals, such as sulfuric acid. Further, the pH adjusting chemicals and dispersing agents are themselves impurities whose presence at higher doses can harm the rheological characteristics (viscosity) of the final product. All of these constraints add to the cost of beneficiation. The present invention overcomes these constraints, however, and provides a beneficiated kaolin product of comparable or better quality at significant cost and environmental savings.